Country losing territory on North Island to old Britons (5)
I believe the answer is:
iceni
'old britons' is the definition.
(ancient British tribe)
'country losing territory on north island' is the wordplay.
'country losing territory' becomes 'ice' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' says to put letters next to each other.
'north' becomes 'N' (abbreviation).
'island' becomes 'i'.
'ice'+'n'+'i'='ICENI'
'to' is the link.
(Other definitions for iceni that I've seen before include "Old people" , "British people once" , "Boudicca's tribe" , "Very old folk" , "Rebellious tribe" .)